The present invention relates to a method and plating solution for depositing tin in a manner to reduce, minimize or prevent tin whisker growth from such deposits.
The use of a tin or tin alloy deposit has become increasingly important in fabricating electronic circuits, electronic devices and electrical connectors because of the benefits that such deposits provide. For example, tin and tin alloy deposits protect the components from corrosion, provide a chemically stable surface for soldering and maintain good surface electrical contact. There are many patents that disclose how to apply tin or tin alloy deposits using a variety of plating solutions and methods. Such deposits are typically produced by electroless plating or electroplating.
Regardless of the deposition process employed, it is desirable to form smooth and level deposits of tin on the substrate in order to minimize porosity. It is also desirable to form a coating having a relatively constant thickness in order to minimize etching problems. Furthermore, other problems must be avoided in order to obtain an acceptable deposit. When pure tin is used and is applied to a copper or copper alloy substrate, the resulting deposit suffers from interdiffusion and formation of copper-tin compounds. While these copper-tin compounds can be brittle and may impair the usefulness of the tin coated component, their presence also adversely affect subsequent soldering operations due to the generation of metal filaments known as tin whiskers which sometimes grow spontaneously from these tin deposits. These whiskers are hair-like projections extending from the surface and may be either straight or curled or bent. The presence of such whiskers is undesirable due to the very fine line definition required for modem circuitry, since these whiskers can form both electrical shorts and electrical bridges across insulation spaces between conductors.
The mechanics of the tin whisker problem are not clearly understood. The filaments can begin to grow within days of the application of the coating or even several years thereafter. There is speculation in the literature that the whiskers grow from stress concentration sites, such as those created in many electrodeposition techniques, as tin extrusions which are dendritic in nature. There is also speculation that temperature and humidity affect whisker growth. The article “Simultaneous Growth of Whiskers on Tin Coatings: 20 Years of Observation,” by S. C. Britton, Transactions of the Institute of Metal Finishing, Volume 52, 1974, pp. 95-102 discusses the tin whisker growth problem and offers several recommendations for reducing the risk of whisker formation.
One approach for handling the tin whisker problem has been to specify short storage times for tin coated materials. However, this approach does not fully address or necessarily avoid the problem. Another approach has been to mildly strengthen the tin matrix to prevent extrusion of the whiskers. The formation of an intermetallic compound and diffusion of solute copper into the tin plate have served this purpose but at prohibitive performance cost in the final product.
Another approach is to treat the surface of the substrate before applying the tin deposit. Ultrasonic agitation of the plating solution and/or alternating the polarity of the electrodes during plating have been suggested to reduce the amount of hydrogen absorbed or occluded in the structure of the plating metal. Alternatively, one or more barrier layers or metals such as palladium, gold, silver, nickel, and/or copper can be used to prevent metal ion migration from the substrate into the tin deposit, thus reducing the stress on the deposit. These processes are undesirable due to the additional process steps required as well as due to the high cost of precious metals when they are used. Furthermore, the plating solutions for the barrier layer can sometimes contaminate or otherwise cause interference with the tin plating process.
Recent publications have indicated that tin deposited from methane sulfonic acid (MSA) solutions over copper/copper alloy substrates in the as-plated condition generally start out with no or slightly low compressive stress but during deposit aging compressive stress increases significantly. It is theorized that this increase in compressive stress is due to the formation of copper-tin intermetallic compounds, due to diffusion of copper from the base material into the tin deposit, and that this compressive stress generates tin whisker formation.
Additional approaches for dealing with this problem have generally involved a whisker inhibiting addition to the tin coating solution. A number of different tin-alloying metals including antimony, cobalt, copper, germanium, gold, lead and nickel have been suggested to reduce the growth of tin whiskers in the resulting deposit. In order to avoid the high cost of precious metals, the most common approach has been to deposit an alloy of tin and lead. This alloy is also compatible with the solders that are later used to make electrical connections to wires or other electrical components. Unfortunately, lead and a number of other alloying elements are undesirable due to their toxicity and related environmental issues.
Thus, an effective solution to the tin whisker problem still remains, and one is provided herewith.